Hybrid (passive/active) devices have been developed to reduce the magnitudes of possibly threedimensional vibrations coupled from vibrating equipment on platforms to bases that support the platforms. These hybrid devices are three-axis generalizations of singleaxis hybrid vibration reducers developed and reported previously. Each hybrid device includes rubber passive vibration-isolator pads, piezoelectric actuators, optional piezoelectric force sensors, and accelerometers. Each device operates in conjunction with a computer and control-and-actuation circuitry, which process accelerometer readings using feedforward and/or feedback control laws to excite the piezoelectric actuators with signals having amplitudes and phases chosen to minimize acceleration of the base.

A Hybrid Vibration Reducer of the type described in the text includes passive vibration-suppressing components and active components. The device is located between a top platform and a supporting base. Standoffs may be used to facilitate positioning.
One such device, depicted in simplified form in the figure, includes an inner frame, an upper outer frame, and a lower outer frame that serves as a base. The upper outer frame is connected to the lower outer frame via rubber pads. The inner frame holds five piezoelectric actuators: one for motion along the vertical axis and two pairs for motions along the two horizontal axes. The vertical-axis piezoelectric actuator supports a standoff that supports the platform. The upper outer frame supports four rubber pads connected to the horizontalhorizontal- axis piezoelectric actuators, which, in turn, support the inner frame.

Accelerometers are mounted on the base and on the platform. The piezoelectric actuators are excited via electrical cables (not shown in the figure): one cable for the vertical-axis actuator and one cable for the pair of actuators for each horizontal axis. The actuators for each axis are wired in an opposing-polarity, push-pull arrangement: the application of a voltage of a given polarity causes one actuator to expand and the other to contract along the affected axis.

Experiments have confirmed that with proper design, it is possible to achieve a significant reduction in vibration amplitude. For example, for one such device designed to perform optimally at frequencies from 50 to 500 Hz, in tests at 50 Hz, vibrations were suppressed by amounts ranging from about 35 to about 40 dB. Moreover, even when the piezoelectric actuators were not used (that is, when only rubber pads provided the vibration isolation), vibrations were suppressed by about 11 dB.

This work was done by Robert D. Corsaro and Brian H. Houston of the Naval Research Laboratory and Peter C. Herdic of SFA, Inc. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp  under the Mechanics/Machinery category. NRL-0001

This Brief includes a Technical Support Package (TSP).
Hybrid Three-Axis Vibration Reducers

(reference NRL-0001) is currently available for download from the TSP library.

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